Alzheimer's disease (AD) is a progressive neurodegenerative disease of the central nervous system and the most common form of irreversible dementia. It is generally characterized by a slow but inflexible progression of dementia, associated with cognitive and memory decline, speech loss and personality changes. The memory loss is characteristic of AD, firstly a gradual loss of short-period memory occurs, which ultimately is extended to the more consolidated memory.
The prodromal phase of AD is characterized by the appearance of cognitive deficits, depression and functional impairment, while in the early stage of AD a mild cognitive impairment emerges.
Two kinds of hallmark pathological lesions occur, consisting on both amyloid plaques and neurofibrillary tangles in the brain memory and cognition regions.
Amyloid beta peptide (Aβ), a highly insoluble peptide with high tendency to oligomerize and aggregate, is the primary component of the amyloid plaques. The formation of Aβ occurs due to the sequential cleavage of the amyloid precursor protein by β-secretase and γ-secretase. From the proteolytic fragmentation of amyloid precursor protein (APP), several isoforms of Aβ peptide are released, among which Aβ42, a very low soluble isoform forty two amino acid long.
Neurofibrillary tangles are formed by the accumulation of abnormal filaments of tau protein. Tau is a soluble microtubule-binding protein, which supports axonal transport and cytoskeleton growth, by stabilizing microtubules and promoting tubulin assembly into microtubules. Hyperphosphorylation of tau proteins in AD causes the detachment of tau proteins from the microtubule. The soluble tau proteins may then aggregate into soluble tau aggregates and insoluble paired helical filaments that ultimately end in the formation of neurofibrillary tangles. This microtubule destabilization caused by direct toxic effects of both soluble hyperphosphorylated tau and fibrillar tau leads to axonal transport impairment causing a progressive loss of neurons. Aβ was referred to cause hyperphosphorylation of tau proteins turning it responsible for neurofibrillary tangles occurrence.
The excessive production of Aβ in the human body causes its accumulation and deposition contributing to the pathological development of the disease that not only occurs in AD but also in other diseases.
Accumulation of Aβ42 in the spinal cord motor neurons plays a role in the pathogenesis of neurodegeneration in Amyotrophic lateral sclerosis.
The most common form of cerebral amyloid angiopathy worldwide is associated with fibrillar amyloid deposition of Aβ in brain blood vessels. In addition, cerebral amyloid angiopathy is itself a risk factor for the development of cerebral ischemia.
Hereditary cerebral haemorrhage with amyloidosis of the Dutch-type is an autosomal dominant hereditary disease in the amyloid precursor protein gene leading to altered Aβ cleavage and secretion resulting in Aβ accumulation in cerebral vessels, leading to haemorrhages and infarcts.
Abnormal accumulations of Aβ in human muscle found by Askanas et al., 1992 in inclusion body myositis patients emphasises the role of Aβ in the pathogenesis of diseases outside the central nervous system defined as peripheral amyloidoses.
More than 70% of familial Creutzfeldt-Jakob's disease, a prion disorder, possesses an E200K mutation, a nonconservative substitution of lysine for glutamate at codon 200, which has been described to increase Aβ deposition, supporting the idea of cooperative interaction with prion protein to result in disease.
The development of Alzheimer's disease by Down's syndrome patients at the age of 40 years is associated with the accumulation of deposits of Aβ due to an overexpression of amyloid precursor protein. In addition, Parkinson's disease (PD)-related dementia was also found to be associated with AD-pathology.
Head trauma, acquired immunodeficiency syndrome and stroke may play an important role in the pathogenesis of Alzheimer's disease since the deposition of Aβ has been observed in patients suffering severe head trauma and in HIV positive patients, also, patients that suffered acute ischemic stroke showed increased levels of circulating Aβ.
Agents lowering the production of Aβ, through the inhibition of β- or γ-secretase may block or delay AD progression and also other diseases, where excessive production of Aβ in the human body may lead to Aβ accumulation and deposition.
Particularly, pharmaceutical agents designed to inhibit β-secretase should decrease Aβ levels and consequently reduce the occurrence of amyloid plaques and neurofibrillary tangles.
Peptidomimetic β-secretase 1 inhibitors have shown to be ineffective in the treatment of Alzheimer's disease, despite their in vitro high potency inhibition of β-secretase 1. This kind of inhibitors lacks the required pharmacokinetic properties to cross the Blood-Brain Barrier (BBB). Their large size and high hydrophilicity were pointed as the main factor limiting the BBB crossing by diffusion.
Peptidomimetics are compounds whose pharmacophore mimics a natural protein or peptide by retaining their capacity to interact with a specific biological target.
OM00-3, was developed, which is one of the most potent ever reported BACE-1 inhibitors. This inhibitor is composed of natural amino acids plus an isostere moiety, a hydroxyethylene Leucine-Alanine dipeptide transition-state isostere, furnishing a non-cleavable carbon-carbon linkage for BACE-1 inhibition. OM00-3 has been optimized for the inhibition of BACE-1 for which the preference index of each amino acid has been reported. However OM00-3 is not permeable through the BBB due to his large size and hydrophilicity.
Progress led to the development of non-peptidic inhibitors. Despite being much smaller and hydrophobic in relation to peptidomimetic inhibitors, which are good properties to allow permeation through the BBB, efflux by P-glycoprotein has been reported, which limits the achievement of clinically relevant concentrations in the central nervous system. Also their small size contributes to increased toxicity due to nonspecificity.
BACE-1 inhibitors may also inhibit beta-secretase 2 (BACE-2), which has high homology with BACE-1, but is particularly important to avoid its inhibition since it may favour AD. On the other hand BACE-2 inhibitors may be used to induce expansion of functional pancreatic β cell mass for the treatment of type 2 diabetes.
Presently there is no effective treatment capable of modifying the progression of Alzheimer's disease. Currently available drugs only act on symptomatic improvement, while the development of drugs capable of blocking or delaying the disease progression remains a challenging unmet need, mainly due to β-secretase 1 inhibitors limitation in crossing the BBB.
A vascular theory for the development of Alzheimer's disease claims that a substantial amount of Aβ peptide in the brain of Alzheimer's disease patients is originated in the systemic circulation. In addition it was found that Aβ peptide transport across the BBB is mediated through transcytosis by interaction with the receptor for advanced glycation end products (RAGE). Transcytosis is a mechanism allowing the mediation of macromolecules transport from one side of a biological membrane onto the other side, by means of intracellular vesicles formation. In line with this, a binding region was identified in the Aβ peptide, responsible for its recognition by RAGE. This is a seven amino acid fragment of residues consisting on the amino acid sequence between residue number seventeen to the twenty third of Aβ peptide: LVFFAED [Aβ (17-23)] (SEQ ID No: 1). This sequence has also been proved to competitively inhibit the entrance of Aβ peptide into cells expressing RAGE. Also it was concluded that a sequence to recognize RAGE should have a highly hydrophobic segment flanked by two negatively charged residues at the C terminus Aβ (17-23).
In summary, there is yet a need for a strategy with an effective delivery of a BACE-1 inhibitor to the target organ, the brain. The present application comprises a specific drug design that simultaneously inhibits β-secretase 1 and also promotes brain permeation and the achievement of therapeutic concentrations in the brain, through the conjugation and/or combination of peptidomimetic β-secretase 1 inhibitors with an Aβ (17-23) ligand, known to bind RAGE receptor.